In 2000, IEC accepted the IEC 61508 standard (“Functional Safety of Electrical/Electronic/Programmable Electronic Safety-Related System”) to support companies that use Safety Instrumented Systems (SIS) to protect persons and facilities from hazardous events. Four safety integrity levels (SIL, level 1˜4) are defined by IEC 61508 to statistically represent the integrity of the SIS when a process demand occurs. The SIL takes into account device integrity, architecture, voting, diagnostics, systematic and common cause failures, testing, operation, and maintenance. A SIL establishes an order of magnitude targets for risk reduction. This target failure measure is the intended probability of dangerous mode failures to be achieved in respect of the safety integrity requirements, specified in terms of either the average probability of failure to perform the safety function on demand (for a low demand of operation); or the probability of a dangerous failure per hour (for a high demand or continuous mode of operation). The higher the SIL, the greater the impact of a failure and therefore the lower the failure rate that is acceptable. The method used to derive the SIL designation, must be carefully documented using well-established methods. The first step to determine or designate a SIL, is to conduct a process hazards analysis (PHA). The methodology of IEC 61508 focuses most of the actual evaluation on the potential injury, fatality, or other risk to individual persons.
In an environment where articles need to be adjusted using a wired or wireless remote control, actuators may be used switched by relays. Especially when the articles comprise hospital patient table, safety concerns take a high priority.
In a typical embodiment, a micro-controller scans the inputs of a keyboard of a wireless remote control and/or a footswitch that is used to control a movable patient table. When a person controls the remote an output of its microprocessor will control a relay. Typically, a micro-controller in a remote control could get in an error situation. When an output would get stuck the patient table could then move uncontrollable something that can be very dangerous and must be prevented.
When a micro-controller is connected to a relay an error analysis is difficult to perform as semiconductors and software are involved. For this reason an error or a fault condition is not always detectable. In case of safety relevant applications, this is a serious problem. When an error condition would occur however, it is desirable that any movement will stop immediately to prevent any damage, or worse, patient harm.
It is also desirable to use components in a fail-safe system that are known from the past and that have a well known (reliable) behaviour.